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well my walbro 450 pump showed.....inlet id is 3/8" however outlet od is 3/8". there site said 3/8 x 3/8" id inlet and outlet. once i snap the screen in it then the inlet will match outlet. i def agree about fuel filters and anytime you can filter at 4 microns to 10 microns the better! now those inline aem and knock offs are ss screen and rated at 60 or 100 and not worth a crap really unless space is the deciding factor then i get it. they don't have the flow rating the fst system has nor the filter rating. http://www.speedwaymotors.com/FST-RF500M-Replacement-Fuel-Filter-for-Flo-Max-Systems,112433.htmli know what your saying about fuel line size and od vs id does make a diff esp when multiple connectors or connections jump up and down on id sizes creating restrictions or even length of the line and is 2' vs 22'. psi vs flow of the pump are a huge part of supplying enough fuel as we all know and diff pumps have diff flow ratings. but all these variances are what dictate needing 1/2" or 3/8" feed line.
1/2 inch feed lines with 3/8 return is more than adequate.
You always want your return line larger than your feed line.
carbs don't have return lines.My race carbs were set up with the regulator AFTER the carb inlets and use a return line. Just because it is a carb setup doesn't mean you can't plumb it with a return line. fuel injection (FI) do because of the higher psi and flow.It has nothing to do with injection. FI can be setup just fine as a returnless system. Any aftermarket FI can be setup both ways. It makes zero difference to the injectors. It DOES make a difference to the pump though since the continous flow through the pump keeps it cool. The FAST EFI I just installed has provision for a solid state relay that continuously cycles the pump to maintain consistent pressure without a regulator using just a pressure sensor if a person wished. I wouldn't but,it can be done that way.Much harder on the pump. for anyone that says my bike is FI and only has a feed line, you are wrong. it has "a" feed line and a return port that is built into the pump housing that you cannot see. now bdkw1 ........i have always ran 1 step smaller id return lines simply because the reg isn't returning the same amount of fuel the pump is delivering nor even at the same psi. what ( in your experiance) has/did/does it do to the regulator?fabbr i have actually read those # on fuel line size on there site but saw no reference to fuel type. Those numbers were for gas. Using E85 would require a bit of adjustment as you know. Yes,I would go 1/2" feed though as it will not harm a thing.As for return,like you,I use 3/8" but Aeromotive says to stay same as feed. Makes sense as bdkw points out though even though I've never seen an issue with a next size smaller return.we all know the we need larger injectors as we consume more of the alky fuel types so it would stand to reason imo that we need a slightly larger feed line for the hp range. now that being said where you pasted up to 600 hp 3/8" is the spec from aem....where i will have to cap at 400 hp for my trans to even hope to live, even on e85 tech i should be fine at 3/8" but if poss i would feel better at 1/2". personal choice however the simplest route with the least amount of adaptors/couplers is of course how i will go.